The present invention relates to variable ratio transmission or drive system generally and to bicycles equipped with such mechanisms.
Bicycles equipped with chain-driven multi-speed transmissions called "derailleurs" have become popular world-wide. Derailleurs enable the bicycle rider to select various "speeds" for various slopes of the road and to accommodate the different needs of vigorous and weaker riders. A selected "speed" of a bicycle transmission identifies the ratio of the number of turns of the pedal crank to the resulting number of turns of the driven rear wheel.
A vigorous rider will choose a high speed or transmission ratio for riding down-hill or along a level road so that pedalling at a reasonable rate causes the bicycle to travel fast, and the same rider will choose a different ratio, a low speed, when riding slowly up-hill or on a poor road such as gravel.
Derailleurs with ten speeds are most common. A rider may wish that there were additional choices between the speeds of the ten-speed transmission, but derailleurs having more than ten speeds are uncommon because of increased cost, weight and troublesome complexity. A derailleur is inherently heavy due to its chain and many gears which are of metal, whereas bicycle riders have high regard for lightweight bicycles. Derailleurs lack an indicator to tell the rider which speed or ratio has been selected. And most derailleurs lack a housing so that the mechanism is exposed to rain, abrasive dirt, and other contaminants, leading to rust, wear and other damage. Finally, when a bicycle is equipped with a derailleur, it is difficult and cumbersome to remove and reassemble the rear wheel of the bicycle for replacing or repairing the tire. It is equally difficult to remove the whole transmission for replacement.
An alternative type of variable-ratio transmission for bicycles is disclosed in U.S. Pat. Nos. 4,030,373 and 4,530,676 in which I am the inventor. Both of these patents are incorporated herein by reference. That type of transmission includes a pair of adjustable sheaves coupled to each other by a belt. The present invention involves a novel transmission of that type.
In U.S. Pat. No. 4,030,373, a set of sheave segments are continuously adjustable radially on each of the two sheaves, so that fine locking teeth are required to maintain any adjustment. Additionally, the mechanism which locks each segment in any selected adjustment is made of tiny parts that are contained within each sheave segment. In practice, such tiny parts are preferably made of metal by die-casting; consequently, the parts are relatively expensive, and their large weight produce centrifugal forces. Manufacture of the rotary member can also be critical because warping of the rotary member under some circumstances may interfere with the necessary cooperation between an unlocking device which is disposed adjacent to the sheave at various distances from its axis and the locked sheave segments which are carried past the unlocking device as the sheave rotates.
In the transmission in U.S. Pat. No. 4,530,676, each of the sheaves also has a set of sheave segments that are adjustable. As an improvement over the '373 patent, the adjustments in '676 differ by discrete increments. A large number of discrete transmission ratios can be provided and relatively large locking teeth are used which are capable of withstanding much greater forces imposed on the sheave segments by the belt than in the '373 patent. Either of the sheaves in '676 can be used as the driving sheave in general applications of the transmission.